Diazotization is a reaction between a primary aromatic amine and nitrous acid to give a diazo compound. The most common method of diazotizing a primary aromatic amine is by slow addition of an aqueous solution of sodium nitrite to a solution of the amine and dilute mineral acid held at 0.degree. to 10.degree. C. Not only can amino groups on benzene, naphthlene, and their substituted derivatives be diazotized, but heterocylic amines such as amino thiazoles and certain amino pyridines will also undergo the reaction. There are many detailed reviews of diazo chemistry, for example, Ullmann's Encyclopedia of Industrial Chemistry, Vol. A8, page 505 (VCH Publishers, New York, N.Y.).
Sodium nitrite, a primary diazotizing reagent, is quite stable by itself. The formed diazo compounds or salts, however, are relatively unstable but are very useful in other reactions. The diazo group on the aromatic ring may readily be replaced with moieties or radicals such as bromo, chloro, cyano and other groups. Diazo salts will couple to active sites on the ring of activated aromatic compounds such as phenols, naphthols, amines and certain other compounds to form diazo dyes. This reaction is the basis of diazo chemistry's most important commercial use. It is generally believed that the reaction mechanism is based on the attack of the strongly electrophilic diazo ion on a position of high electron density of a nucleophillic coupling component.
Electronegative substituents on the ring of the diazo compound increases its coupling power and the diazo compound of trinitroaniline, which is the most reactive diazo compound in this respect, will couple even with mesitylene. Diazo compounds can also couple with phenol and naphthyl ethers, compounds containing reactive methylene groups such as acetoacetanilides, nitriles, pyrazolinones, nitroparaffins, certain unsaturated hydrocarbons, and heterocyclic compounds of high electron density.
In the case of disperse dyes, the diazo base, that is, the primary aromatic amine that is to be diazotized is most often aniline with one to three strongly electron withdrawing substituents in the 2, 4 or 6 positions. The 4 position is usually the nitro group and the 2 and 6 positions are generally occupied by either hydrogen, nitro, chloro, bromo or cyano groups. Heterocyclic compounds such as substituted 2 amino-benzothiazoles, 2 amino thiazoles, and 4 amino pyridines are also used as diazo bases for dispersed dyes. In the case of 4 nitroaniline, and 2 chloro-4 nitro aniline, the diazo salt is formed by heating the amine in water with four or more mole equivalents of hydrochloric acid until the amine dissolves, then cooling to ice temperature. At this point a mole equivalent of sodium nitrite dissolved in ice water is slowly added and the mixture allowed to diazotize.
In the case of aniline with two nitro or cyano groups, aniline with three or more substituents as above, or most heterocyclics, diazotization can no longer be carried out in aqueous solution and is most often carried out with nitrosylsulfuric acid (sodium nitrite dissolved in an excess of sulfuric acid) in sulfuric acid or sulfuric acid in a lower carboxylic acid such as acetic or proprionic. In such a case an appropriate amount of sodium nitrite is dissolved in cold sulfuric acid and added to a solution of the amine in acid. The cold mix is warmed to 40.degree.-60.degree. C. and held until a drop into cold water totally dissolves. This mix is then mixed with crushed ice while stirring rapidly and the resulting cold diazo solution filtered.
Once the diazo salt is formed by either process, it is added to a cold solution of coupling compound (coupler). This coupler is most often a tertiary aromatic amine (for dispersed dyes) which is dissolved in solution of water or water and a solvent but contains sufficient base such as, for example, sodium acetate or sodium carbonate to achieve only slightly acidic pH when mixed with the acetic diazo solution.
After the two solutions are mixed, the dye precipitates and after a time the remaining contaminated salt solution is washed away and the dye filtered off. The dye is then mixed with dispersing agents and subjected to extensive grinding, usually in a sandmill, mixed with more dispersing agents or diluents and then either dried or made into a paste for use.
Though there is nothing very complex about these procedures and though yields are generally good, the waste generated, the washing, the filtering, the grinding, and the mixing all contribute to the cost of the process.